Method of manufacturing a semiconductor integrated circuit and semiconductor integrated circuit

ABSTRACT

Conventionally, when an electric potential of a supporting substrate is fixed, there arises a problem in that impact ions are generated even in the vicinity of embedded insulating film in the proximity of a drain due to generation of a parasitic transistor using the supporting substrate as a gate so as to be likely to cause a parasitic bipolar operation. A method of the present invention includes the steps of: forming and patterning a LOCOS reaching an embedded insulating film, a gate oxide film, a well and a polysilicon film serving as a gate electrode; forming a second conductivity type high-density impurity region in an ultra-shallow portion of each of a source region and a drain region, a second conductivity type impurity region having a low density under the second conductivity type high-density impurity region of the ultra-shallow portion, and a second conductivity type impurity region having a high density under the second conductivity type impurity region having a low density and above the embedded insulating film; forming a sidewall around the gate electrode; forming a second conductivity type impurity region in each of the source region and the drain region; forming an interlayer insulating film and forming contact holes in the source region, the drain region and the gate electrode; and forming a wiring on the interlayer insulating film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a transistorhaving a structure allowing the reduction of impact ionization in atransistor formed on an SOI wafer. In particular, the present inventionrelates to a method of manufacturing an SOI transistor having anelectric potential of a supporting substrate fixed to a GND level or alow voltage level.

2. Description of the Related Art

FIGS. 4A to 4D show a method of manufacturing a conventional SOItransistor, and FIGS. 5A and 5B are a top view and a cross-sectionalview showing a structure of the conventional SO transistor. Herein, thetransistor is formed by using a wafer in which a P-type semiconductorfilm is formed on a P-type supporting substrate through an embeddedinsulating film.

The conventional SOI transistor is formed in a semiconductor film 1surrounded by a LOCOS 11 reaching an embedded insulating film as shownin FIG. 5. The transistors are completely isolated from each other bythe LOCOS 11. In the case of an N-type transistor, since thesemiconductor film 1 is of P-type, a transistor is formed by implantingN-type ions to source/drain regions 14 and 15.

On the other hand, in the case of a P-type transistor, the semiconductorfilm 1 surrounded by the LOCOS 11 is implanted with N-type ions so as tobe imparted with an N-type conductivity. In this state, the source/drainregions 14 and 15 are implanted with P-type ions to form a transistor.

As a manufacturing method, as shown in FIGS. 4A to 4D, first, a nitridefilm 8 is grown. The formed nitride film 8 is patterned and then isthermally oxidized to form the LOCOS 11. The nitride film 8 is oxidizedso that the LOCOS 11 has a thickness reaching the embedded insulatingfilm 2. Next, ion implantation is conducted by using a resist 6 as amask so as to form a well 7 (FIG. 4A). At this point, an energy of theion implantation is controlled so as to have a density peak in thesemiconductor film.

Next, a thermal treatment is conducted so as to activate and diffuse theimplanted ions. After formation of the LOCOS 11, the formation of a gateoxide film 13, the formation of a gate electrode 12, and the ionimplantation to the source/drain regions 14 and 15 of the transistor areperformed. Then, an interlayer insulating film 18 is formed (FIG. 4C).Furthermore, the interlayer insulating film 18 is patterned and etchedto form contacts 19 to the gate electrode 12 and the source/drainregions 14 and 15. Then, a wiring 20 is provided (FIG. 4D).

Since an electric potential of the supporting substrate 3 affects thecharacteristics of the transistor in the case of the SOI transistor, itis necessary to fix the electric potential of the supporting substrate3. Therefore, the electric potential of the supporting substrate 3 isobtained from an electrically conductive pedestal adhered through anelectrically conductive adhesive when the supporting substrate is to bemounted onto a package. Normally, the supporting substrate 3 isconnected to a ground terminal or a power source voltage terminal.

In a conventional method of forming an SOI transistor, since thetransistor formed on the semiconductor film and the supporting substrateare not electrically connected to each other because of the presence ofthe embedded insulating film between the supporting substrate and thesemiconductor film, an electric potential of the supporting substrate isin a floating state. In the case of a fully depleted SOI transistor orthe like, however, the semiconductor film is entirely depleted in itsthickness direction to such a degree that the depletion reaches theembedded insulating film. Therefore, the characteristics of thetransistor are greatly affected by the electric potential of thesupporting substrate. As a result, a variation in the electric potentialof the supporting substrate exhibits similar characteristics as the backgate effect of a bulk transistor.

Thus, it is necessary to fix the electric potential of the supportingsubstrate. A method of fixing the electric potential of the supportingsubstrate is normally conducted by adhering the supporting substrate toan electrically conducive pedestal through an electrically conductiveadhesive upon mounting on a package. In this state, the electricpotential of the pedestal is fixed so as to fix the electric potentialof the supporting substrate. The electric potential of the supportingsubstrate is connected either to a ground terminal or to a power sourcevoltage terminal. In order to fix the electric potential of thesupporting substrate, there is also a method of providing a through holepenetrating through the semiconductor film and the embedded insulatingfilm to reach a part of the supporting substrate.

In the case where the electric potential of the supporting substrate isfixed by the above-described connection methods, a parasitic transistorusing the supporting substrate as a gate is formed. When the electricpotential of the supporting substrate serving as the gate of theparasitic transistor is set to the GND level, a difference in theelectric potential between the gate and a drain is increased. As aresult, impact ionization occurs in the proximity of the drain of abody.

Unlike the SOI transistor, a parasitic transistor is not formed in aconventional bulk transistor. Therefore, although the impact ionizationoccurs in a concentrated manner only in the vicinity of the substratesurface in the proximity of a drain in the conventional bulk transistor,the impact ionization occurs even in the vicinity of the embeddedinsulating film in the proximity of the drain in addition to thevicinity of the substrate surface in the proximity of the drain in theSIO transistor due to formation of a parasitic transistor. The amount ofgenerated impact ions is increased, so that a parasitic bipolarphenomenon, in which a hole of a pair of an electron and a hole flowsinto a source as a bipolar current, is likely to occur in an N-typetransistor. As a result, the operation of the transistor cannot becontrolled by a gate voltage.

As a method of restraining the occurrence of the parasitic bipolarphenomenon, there is a method of setting a body electric potential asshown in FIG. 6 so as to compulsively pull holes out from a body.However, since a layout of the transistor used for a bulk transistor isremarkably different from that used for the SOI transistor, a layoutmodification from a conventional layout becomes a great encumbrance inthe case where a circuit design using the SOI device is to be achieved.Furthermore, in principle, the SOI device has a latchup free structure.Therefore, it is not necessary to provide a guard ring for thetransistor, and thus is greatly effective to reduce the area. In themethod of setting a body electric potential so as to compulsively pullthe holes out from the body, however, the effects of the SOI device ofreducing the area is disadvantageously halved.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method ofmanufacturing a semiconductor integrated circuit, in which a CMOStransistor is formed on a first conductivity type semiconductor filmprovided on a first conductivity type supporting substrate through anembedded insulating film, including the steps of: conducting thermaloxidation so as to reach the embedded insulating film to form a LOCOSfor element separation between transistors; forming a gate oxide film ofa first second conductivity type transistor; forming a firstconductivity type impurity region reaching the embedded insulating filmon the semiconductor film in a region where the first secondconductivity type transistor is to be formed; forming a polysilicon filmserving as a gate electrode of the first second conductivity typetransistor so as to form a second conductivity type impurity region;forming a second conductivity type impurity region in an ultra-shallowportion of each of a source region and a drain region; forming a secondconductivity type impurity region having a low density under the secondconductivity type impurity region in the ultra-shallow portion; forminga second conductivity type impurity region having the same density asthe second conductivity type impurity region in the ultra-shallowportion under the second conductivity type impurity region having thelower density and above the embedded insulating film; forming aninsulating film on the source region, the drain region, and the gateelectrode; dry etching the insulating film formed on the source region,the drain region and the gate electrode so as to form a sidewall aroundthe gate electrode; forming a second conductivity type impurity regionin each of the source region and the drain region; forming an interlayerinsulating film and forming contact holes in the source region, thedrain region, and the gate electrode; and forming a wiring on theinterlayer insulating film.

As a result, in the transistor formed on the semiconductor film, adepletion layer generated by a difference in electric potential betweena drain and a body can be extended toward the body side in a portion ofthe drain region having a high density, whereas the depletion layer canbe actively extended toward the drain side in a portion of the drainregion having a low density. Therefore, the electric potentialconcentration can be reduced in the vicinity of the body surface of theproximity of the drain and in the vicinity of the embedded insulatingfilm, thereby reducing the generation of impact ions.

Further, the SOI transistor has a disadvantage in that a method ofpulling holes generated by impact ionization out from a body terminalhalves the area reduction effect. However, since the generation ofimpact ions themselves is reduced without providing a body terminal inthe SOI transistor according to the present invention, the SOItransistor according to the present invention is effective to realizethe area reduction effect which is an advantage of the SOI device.

Further, according to the present invention, there is provided a methodof manufacturing a semiconductor integrated circuit, in which a CMOStransistor is formed on a first conductivity type semiconductor filmprovided on a first conductivity type supporting substrate through anembedded insulating film, including the steps of: conducting thermaloxidation so as to reach the embedded insulating film to form a LOCOSfor element separation between transistors; forming a gate oxide film ofa first second conductivity type transistor; forming a firstconductivity type impurity region reaching the embedded insulating filmon the semiconductor film in a region where the first secondconductivity type transistor is to be formed; forming a polysilicon filmserving as a gate electrode of the first second conductivity typetransistor so as to form a second conductivity type impurity region;forming a second conductivity type impurity region in an ultra-shallowportion of each of a source region and a drain region; forming a secondconductivity type impurity region having a low density under the secondconductivity type impurity region in the ultra-shallow portion; forminga second conductivity type impurity region having the same density asthe second conductivity type impurity region in the ultra-shallowportion under the second conductivity type impurity region having thelow density and above the embedded insulating film; providing a mask onthe gate electrode and a part of the source region and the drain regionso as to form a second conductivity type impurity region in each of thesource region and the drain region; forming an interlayer insulatingfilm and forming contact holes in the source region, the drain region,and the gate electrode; insulating film and forming a wiring on theinterlayer. In a transistor formed by the above-mentioned method, awidth of the portion of the drain region having a low density in achannel length direction is affected by a width of the mask. Therefore,a width in a channel length direction can be more easily controlled ascompared with the case where a sidewall is provided around the gateelectrode and a portion having a low density is formed in the drainregion. As a result, the extension of the depletion layer in theproximity of the drain can be adjusted so as to be uniform. Accordingly,the impact ionization can be reduced in the vicinity of the body surfacein the proximity of the drain or in the vicinity of the embeddedinsulating film.

Furthermore, according to the present invention, there is provided amethod of manufacturing a semiconductor integrated circuit, in which aCMOS transistor is formed on a first conductivity type semiconductorfilm provided on a first conductivity type supporting substrate throughan embedded insulating film, including the steps of: conducting thermaloxidation so as to reach the embedded insulating film to form a LOCOSfor element separation between transistors; forming a gate oxide film ofa first second conductivity type transistor; forming a firstconductivity type impurity region reaching the embedded insulating filmon the semiconductor film in a region where the first secondconductivity type transistor is to be formed; forming a firstconductivity type impurity region having a higher density than that ofthe first second conductivity type impurity region in a portion of thesemiconductor film serving as the proximal region to a drain in thefirst conductivity type impurity region; forming a polysilicon filmserving as a gate electrode of the first conductivity type transistorand forming a second conductivity type impurity region; forming a secondconductivity type impurity region in each of the source region and thedrain region; forming an interlayer insulating film and forming contactholes in the source region, the drain region, and the gate electrode;and forming a wiring on the interlayer insulating film. In a transistorformed by the above-mentioned method, the depletion layer is extendedtoward the body side in the portion having a low density of the firstconductivity type impurity region in the proximity of the drain whilebeing extended toward the drain side in the portion having a highdensity so as to allow the uniformization of the extension of thedepletion layer in the proximity of the drain. As a result, thegeneration of impact ions can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1F are process flow diagrams (1) showing a firstmanufacturing method of the present invention;

FIGS. 2A to 2F are process flow diagrams (2) showing the firstmanufacturing method of the present invention;

FIGS. 3A and 3B are a top view and a cross-sectional view showing astructure of the transistor of the first manufacturing method of thepresent invention;

FIGS. 4A to 4D are process flow diagrams showing a conventionalmanufacturing method;

FIGS. 5A and 5B are a top view and a cross-sectional view showing astructure of the transistor of the conventional manufacturing method;

FIG. 6 is a top view showing a hole pull-out method of the conventionalmanufacturing method;

FIGS. 7A to 7F are process flow diagrams (1) showing a secondmanufacturing method of the present invention;

FIGS. 8A to 8F are process flow diagrams (2) showing the secondmanufacturing method of the present invention;

FIGS. 9A and 9B are a top view and a cross-sectional view showing astructure of the transistor of the second manufacturing method of thepresent invention;

FIGS. 10A and 10B are a top view and a cross-sectional view showing astructure of a second transistor of the second manufacturing method ofthe present invention;

FIGS. 11A to 11F are process flow diagrams (1) showing a thirdmanufacturing method of the present invention;

FIGS. 12A to 12C are process flow diagrams (2) showing the thirdmanufacturing method of the present invention; and

FIGS. 13A and 13B are a top view and a cross-sectional view showing astructure of the transistor of the third manufacturing method of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a first embodiment of the present invention will bedescribed based on FIGS. 1A to 1F and FIGS. 2A to 2F.

In a method of manufacturing a semiconductor integrated circuitdescribed in the first embodiment of the present invention, a method ofmanufacturing an N-type transistor and a P-type transistor in a P-typesemiconductor film formed on a P-type supporting substrate through anembedded insulating film will be described. The same principle can beapplied to a method of forming a transistor in an N-type semiconductorfilm formed on an N-type supporting substrate through an embeddedinsulating film. More specifically, two cases are conceived: the casewhere a first conductivity type is P-type and a second conductivity typeis N-type; and the case where a first conductivity type is N-type and asecond conductive type is P-type.

Although only an embodiment for an N-type SOI transistor where the firstconductivity type is N-type is described, a P-type SOI transistor formedon the same N-type supporting substrate can be formed by the same methodas that for the N-type transistor so as to provide the completelyreversed conductivity.

Hereinafter, a method of manufacturing a semiconductor integratedcircuit according to the present invention will be described.

As shown in FIG. 1A, an SOI wafer having a semiconductor film 1 providedon a supporting substrate 3 through an embedded insulating film 2 isthermally oxidized so as to form a thermal oxide film 5 having athickness of several hundreds of nm. A nitride film 8 is formed thereonto a thickness of about 1600 nm. Next, a LOCOS 11 is formed. First,alignment and exposure to light are performed for patterning of theLOCOS 11.

Next, the nitride film 8 is etched to form an opening in a LOCOSformation region. The SOI wafer in this state is placed in a thermaloxidation oven so as to form the LOCOS 11 for element isolation betweena P-type transistor formation region 9 and an N-type transistorformation region 10. The LOCOS 11 is formed to have such a thicknessthat the LOCOS 11 reaches the embedded insulating film 2 overlying thesupporting substrate 3. FIG. 1B shows a state where a mask of thethermal oxide film 5 and the nitride film 8 is formed in the P-typetransistor formation region 9 and the N-type transistor formation region10. After formation of the LOCOS 11, the nitride film 8 is removed.Then, all the oxide film other than the LOCOS 11 region is removed so asto conduct a gate oxidation process.

FIG. 1C shows a state where all the oxide film other than the LOCOS 11is removed.

Furthermore, FIG. 1D shows a state after conducting the gate oxidationprocess. The patterning is conducted on a gate oxide film 13 by using aresist 6 so as to form an opening for ion implantation to a well 7.Next, as shown in FIG. 1D, ion implantation is performed through thegate oxide film 13, using the resist 6 as a mask. As a result, ions areimplanted only in the opening formed through the resist 6. At thispoint, the energy for ion implantation is adjusted so as to have a peakof a density distribution in the semiconductor film 1.

Next, as shown in FIG. 1E, after formation of a polysilicon film,alignment and exposure to light are performed for patterning of a gateelectrode 12. Next, the polysilicon film is etched by dry etching so asto form the gate electrode 12.

Next, as shown in FIG. 1F, an insulating film 16 having a thickness ofabout several hundreds of angstrom is formed on an N-type transistorsource region 14, an N-type transistor drain region 15 and the gateelectrode 12. In order to conduct shallow ion implantation, for example,an ultra-shallow high-density N-type source region 141 and anultra-shallow high density N-type drain region 151 having a density ofabout 1E18/cm³ are formed at the energy of about 40 KeV.

Next, as shown in FIG. 2A, for ion implantation to a depth around themiddle portion in the drain region 14 and the source region 15, forexample, a low-density N-type source region 142 and a low-density N-typedrain region 152 having a density of about 1E17/cm³ are formed at theenergy of about 60 KeV.

Furthermore, as shown in FIG. 2B, in order to conduct deep ionimplantation, for example, an embedded insulating film neighboringhigh-density N-type source region 143 and an embedded insulating filmneighboring drain region 153 having a density of about 1E1/cm³ areformed at the energy of about 100 KeV. Next, as shown in FIG. 2C, theinsulating film 16 overlying the source/drain regions and the gateelectrodes is dry etched so as to form a sidewall 17 around the gateelectrode 12. The sidewall 17 serves as an implantation mask when theimplantation to the source and the drain is conducted in the laterprocess.

Next, as shown in FIG. 2D, a high-density N-type source region 144 and ahigh-density N-type drain region 154 having a density of about 1E18/cm³are formed in the source region and the drain region at the energy ofabout 60 KeV. By this implantation, the N-type impurity regions whereonly a part of the source region and the drain region has a low densitycan be formed. From there on, the same steps as those of a normal CMOSmanufacturing process are conducted. As shown in FIG. 2E, an interlayerinsulating film 18 is formed. Then, contacts 19 in the source/drainregions of the transistor are formed.

Next, as shown in FIG. 2F, a metal film is formed, and a resist isapplied thereon. The alignment and the exposure to light are conductedfor patterning and etching of a wiring. Thereafter, a protective film isformed on the wiring. A bonding pad is formed to complete asemiconductor integrated circuit.

FIGS. 3A and 3B are a top view and a cross-sectional view showing astructure of a transistor according to a first manufacturing method ofthe present invention. FIG. 3B is a cross-sectional view, cut along aline A-A1 in the top view of FIG. 3A. Since the same reference numeralsas those in FIGS. 1A to FIG. 2F are used in FIGS. 3A and 3B, thedescription thereof is omitted herein. As shown in FIGS. 3A and 3B,since a depletion layer in the proximity of the drain of the transistorformed can be actively extended toward the drain side, that is, towardthe N-type drain low-density region 152 side in the vicinity of themiddle of the semiconductor film 1, the extension of the depletion layertoward the body side can be restrained. Thus, a width of the depletionlayer in a channel length direction scarcely differs in the vicinity ofthe body surface in the proximity of the drain and the vicinity of theembedded insulating film 2. As a result, the electric field density inthe vicinity of the body surface or the vicinity of the embeddedinsulating film 2 can be restrained. Therefore, the generation of theimpact ions can be reduced.

Furthermore, the SOI transistor has a disadvantage in that the areareduction effect is halved in the case where a method of providing abody terminal and pulling holes generated by the impact ionization outfrom the body terminal is employed. On the other hand, the SOItransistor according to the present invention has the effect of reducingthe generation of impact ions themselves without providing a bodyterminal and losing the area reduction effect which is the advantage ofthe SOI device.

Next, a second embodiment of the present invention will be describedbased on FIGS. 7A to 7F and FIGS. 8A to 8F.

Hereinafter, a method of manufacturing a semiconductor integratedcircuit according to the present invention will be described.

As shown in FIG. 7A, an SOI wafer having the semiconductor film 1provided on the supporting substrate 3 through the embedded insulatingfilm 2 is thermally oxidized so as to form the thermal oxide film 5having a thickness of several hundreds of nm. The nitride film 8 isformed thereon to a thickness of about 1600 nm.

Next, the LOCOS 11 is formed. First, alignment and exposure to light areperformed for patterning of the LOCOS 11. Next, the nitride film 8 isetched to form the opening in the LOCOS formation region. The SOI waferin this state is placed in a thermal oxidation oven so as to form theLOCOS 11. The LOCOS 11 is formed to have such a thickness that the LOCOS11 reaches the embedded insulating film 2 overlying the supportingsubstrate 3. FIG. 7B shows a state where a mask of the thermal oxidefilm 5 and the nitride film 8 is formed in the P-type transistorformation region 9 and the N-type transistor formation region 10. Afterformation of the LOCOS 11, the nitride film 8 is removed. Then, all theoxide film other than the LOCOS 11 is removed so as to conduct a gateoxidation process.

FIG. 7C shows a state where all the oxide film other than the LOCOS 11is removed.

Furthermore, FIG. 7D shows a state after conducting the gate oxidationprocess. The patterning is conducted on the gate oxide film 13 by usingthe resist 6 so as to form an opening for ion implantation to the well7. Next, as shown in FIG. 7D, ion implantation is performed through thegate oxide film 13, using the resist 6 as a mask. As a result, ions areimplanted only in the opening formed through the resist 6. At thispoint, the energy for ion implantation is adjusted so as to have a peakof a density distribution in the semiconductor film 1.

Next, as shown in FIG. 7E, after formation of the polysilicon film,alignment and exposure to light are performed for patterning of the gateelectrode 12. Next, the polysilicon film is etched by dry etching so asto form the gate electrode 12.

Furthermore, as shown in FIG. 7F, in order to conduct shallow ionimplantation, for example, the ultra-shallow high-density N-type sourceregion 141 and the ultra-shallow high-density N-type drain region 151having a density of about 1E18/cm³ are formed at the energy of about 40KeV.

Next, as shown in FIG. 8A, for ion implantation to a depth around themiddle portion in the drain region 14 and the source region 15, forexample, the low-density N-type source region 142 and the low-densityN-type drain region 152 having a density of about 1E17/cm³ are formed atthe energy of about 60 KeV.

Furthermore, as shown in FIG. 8B, in order to conduct deep ionimplantation, for example, the embedded insulating film neighboringhigh-density N-type source region 143 and the embedded insulatingneighboring drain region 153 having a density of about 1E18/cm³ areformed at the energy of about 100 KeV. Next, unlike the firstembodiment, the resist 6 is applied onto the gate electrode 12, thesource region 14 and the drain region 15 as shown in FIG. 8C in thissecond embodiment. The alignment and the exposure to light are conductedto form a mask on the gate electrode 12, and a part of the source region14 and the drain region 15.

Then, as shown in FIG. 8D, the high-density N-type source region 144 andthe high-density N-type drain region 154 having a density of about1E18/cm³ are formed in the source region and the drain region at theenergy of about 60 KeV. By this implantation, the N-type impurityregions where only a part of the source region and the drain region hasa low density can be formed,

From there on, the same steps as those of a normal CMOS manufacturingprocess are conducted as in the first embodiment. As shown in FIG. 8E,the interlayer insulating film 18 is formed. Then, the contacts 19 inthe source/drain regions of the transistor are formed. Next, as shown inFIG. 8F, a metal film is formed, and a resist is applied thereon. Thealignment and the exposure to light are conducted for patterning andetching of the wiring 20. Thereafter, a protective film is formed on thewiring 20. A bonding pad is formed to complete a semiconductorintegrated circuit.

FIGS. 9A and 9B are a top view and a cross-sectional view showing astructure of a transistor according to the second manufacturing methodof the present invention. FIG. 9B is a cross-sectional view, cut along aline A-A1 in the top view of FIG. 9A. Since the same reference numeralsas those in FIGS. 7A to FIG. 8F are used in FIGS. 9A and 9B, thedescription thereof is omitted herein. As shown in FIGS. 9A and 9B,since a depletion layer in the proximity of the drain of the transistorformed can be actively extended toward the drain side, that is, towardthe N-type drain low-density region 152 side in the vicinity of themiddle of the semiconductor film 1, the extension of the depletion layertoward the body side can be restrained. Thus, a width of the depletionlayer in a channel length direction scarcely differs in the vicinity ofthe body surface in the proximity of the drain and the vicinity of theembedded insulating film 2. As a result, the electric field density inthe vicinity of the body surface or the vicinity of the embeddedinsulating film 2 can be restrained. Therefore, the generation of theimpact ions can be reduced.

In the case where the sidewall 17 is provided around the gate electrode12 so as to serve as a mask upon implantation to the source region 14and the drain region 15 in the embodiment 1, a width of the N-type drainlow-density region is about 0.1 micron. On the other hand, in thissecond embodiment, high-density implantation to the source region 14 andthe drain region 15 is performed by providing a mask on the gateelectrode 12 and a part of the source region 14 and the drain region 15.Therefore, since a width 21 of the N-type drain low-density region canbe adjusted by a width of the mask, the extension of the deletion layertoward the body side can be uniformized from the vicinity of the bodysurface to the vicinity of the embedded insulating film.

The N-type low density region is required only for the drain side and isnot needed for the source side where impact ions are not generated.FIGS. 10A and 10B are a top view and a cross-sectional view showing astructure of a second transistor according to the second manufacturingmethod of the present invention. In the case where high-densityimplantation to the source region 14 and the drain region 15 isperformed with a mask being provided on the gate region 12, and a partof the source region 14 and the drain region 15, an N-type low-densityregion can be provided only on the drain side as shown in FIG. 10B.

Next, a third embodiment of the present invention will be describedbased on FIGS. 11A to 11F and FIGS. 12A to 12C. Hereinafter, a method ofmanufacturing a semiconductor integrated circuit according to thepresent invention will be described.

As shown in FIG. 11B, the LOCOS 11 is formed in an SOI wafer having thesemiconductor film 1 provided on the supporting substrate 3 through theembedded insulating film 2. First, the thermal oxide film 5 is formed toa thickness of several hundreds of angstrom. The nitride film 8 isformed thereon to a thickness of about 1600 angstrom. Next, alignmentand exposure to light are performed for patterning of the LOCOS 11.Then, the nitride film 8 is etched to form the opening in the LOCOSformation region. FIG. 11A shows a state where a mask of the thermaloxide film 5 and the nitride film 8 is formed on the P-type transistorregion 9 and the N-type transistor region 10. The SOI wafer in thisstate is placed in a thermal oxidation oven so as to form the LOCOS 11for element separation between the P-type transistor formation region 9and the N-type transistor formation region 10 as shown in FIG. 11B. TheLOCOS 11 is formed to have such a thickness that the LOCOS 11 reachesthe embedded insulating film 2 overlying the supporting substrate 3.After formation of the LOCOS 11, the nitride film 8 is removed. Then,all the oxide film other than the LOCOS 11 is removed so as to conduct agate oxidation process to form the gate oxide film 13 (FIG. 11C).

Then, the patterning is conducted by using the resist 6 formed on thegate oxide film 13 so as to form an opening for ion implantation to thewell 7. Next, as shown in FIG. 11D, ion implantation is performedthrough the gate oxide film 13, using the resist 6 as a mask. As aresult, ions are implanted only in the opening formed through the resist6. At this point, the energy for ion implantation is adjusted so as tohave a peak of a density distribution in the semiconductor film 1.

Next, the ion implantation is performed through the mask having anopening in the region corresponding to the proximity of the drain in thewell, so that the P-type impurity region 22 having a higher density thanthe well is formed at the middle depth of the semiconductor film byphotolithography and ion implantation.

Furthermore, after formation of a polysilicon film, the alignment andexposure to light are conducted for patterning of the gate electrode 12as shown in FIG. 11F. Next, the polysilicon film is etched by dryetching to form the gate electrode 12. Next, as shown in FIG. 12A, thehigh-density N-type source region 144 and the high-density N-type drainregion 154 having a density of about 1E18/cm³ are formed in the sourceregion and the drain region at the energy of about 60 KeV. From thereon, the same steps as those of a normal CMOS manufacturing process areconducted as in the first embodiment. As shown in FIG. 12B, theinterlayer insulating film 18 is formed. Then, the contacts 19 in thesource/drain regions of the transistor are formed. Next, as shown inFIG. 12C, a metal film is formed, and a resist is applied thereon. Thealignment and the exposure to light are conducted for patterning andetching of the wiring 20. Thereafter, a protective film 19 is formed onthe wiring 20. Furthermore, a bonding pad is formed to complete asemiconductor integrated circuit.

FIGS. 13A and 13B are a top view and a cross-sectional view showing astructure of a transistor according to a third manufacturing method ofthe present invention. FIG. 13B is a cross-sectional view, cut along aline A-A1 in the top view of FIG. 13A. Since the same reference numeralsas those in FIGS. 11A to FIG. 12C are used in FIGS. 13A and 13B, thedescription thereof is omitted herein. As shown in FIGS. 13A and 13B,the depletion layer in the proximity of the drain of the transistor hasa high body density in the vicinity of the middle of the semiconductorfilm 1.

Thus, the extension of the depletion layer toward the body side can berestrained. As a result, a width of the depletion layer in a channellength direction scarcely differs in the vicinity of the body surface inthe proximity of the drain and the vicinity of the embedded insulatingfilm 2. Therefore, the electric field density in the vicinity of thebody surface or the vicinity of the embedded insulating film 2 can berestrained. Consequently, the generation of the impact ions can bereduced.

The present invention is carried out in the modes as described above andhas the effects described as follows.

A method of manufacturing a semiconductor integrated circuit, in which aCMOS transistor is formed on a first conductivity type semiconductorfilm provided on a first conductivity type supporting substrate throughan embedded insulating film, includes the steps of: conducting thermaloxidation so as to reach the embedded insulating film to form a LOCOSfor element separation between transistors; forming a gate oxide film ofa first second conductivity type transistor; forming a firstconductivity type impurity region reaching the embedded insulating filmon the semiconductor film in a region where the first secondconductivity type transistor is to be formed; forming a polysilicon filmserving as a gate electrode of the first second conductivity typetransistor so as to form a second conductivity type impurity region;forming a second conductivity type impurity region in an ultra-shallowportion of each of a source region and a drain region; forming a secondconductivity type impurity region having a low density under the secondconductivity type impurity region in the ultra-shallow portion; forminga second conductivity type impurity region having the same density asthe second conductivity type impurity region in the ultra-shallowportion under the second conductivity type impurity region having thelower density and above the embedded insulating film; forming aninsulating film on the source region, the drain region, and the gateelectrode; dry etching the insulating film formed on the source region,the drain region, and the gate electrode so as to form a sidewall aroundthe gate electrode; forming a second conductivity type impurity regionin each of the source region and the drain region; forming an interlayerinsulating film and forming contact holes in the source region, thedrain region, and the gate electrode; and forming a wiring on theinterlayer insulating film. As a result, in the transistor formed on thesemiconductor film, a depletion layer generated by a difference inelectric potential between a drain and a body can be extended toward thebody side in the portion of the drain region having a high densitywhereas the depletion layer can be actively extended toward the drainside in the portion of the drain region having a low density. Therefore,the electric field density in the vicinity of the body surface of theproximity of the drain or the vicinity of the insulating film can bereduced to reduce the generation of impact ions. Furthermore, the SOItransistor conventionally has a disadvantage in that the use of a methodof pulling holes generated by impact ionization out from a body terminalhalves the area reduction effect. Since the generation of impact ionsthemselves is reduced without providing a body terminal in the SOItransistor according to the present invention, the SOI transistoraccording to the present invention is effective to realize the areareduction effect which is the advantage of the SOI device.

Furthermore, a method of manufacturing a semiconductor integratedcircuit, in which a CMOS transistor is formed on a first conductivitytype semiconductor film provided on a first conductivity type supportingsubstrate through an embedded insulating film, includes the steps of:conducting thermal oxidation so as to reach the embedded insulating filmto form a LOCOS for element separation between transistors; forming agate oxide film of a first second conductivity type transistor; forminga first conductivity type impurity region reaching the embeddedinsulating film on the semiconductor film in a region where the firstsecond conductivity type transistor is to be formed; forming apolysilicon film serving as a gate electrode of the first secondconductivity type transistor so as to form a second conductivity typeimpurity region; forming a second conductivity type impurity region inan ultra-shallow portion of each of a source region and a drain region;forming a second conductivity type impurity region having a low densityunder the second conductivity type impurity region in the ultra-shallowportion; forming a second conductivity type impurity region having thesame density as the second conductivity type impurity region in theultra-shallow portion under the second conductivity type impurity regionhaving the low density and above the embedded insulating film; providinga mask on the gate electrode and a part of the source region and thedrain region so as to form a second conductivity type impurity region ineach of the source region and the drain region; forming an interlayerinsulating film and forming contact holes in the source region, thedrain region, and the gate electrode; insulating film and forming awiring on the interlayer. In a transistor formed by the above-mentionedmethod, a width of the portion of the drain region having a low densityin a channel length direction is affected by a width of the mask.Therefore, a width in a channel length direction can be more easilycontrolled as compared with the case where a sidewall is provided aroundthe gate electrode and a portion having a low density is formed in thedrain region. As a result, the extension of the depletion layer in theproximity of the drain can be adjusted so as to be uniform. Accordingly,the impact ionization can be reduced in the vicinity of the body surfacein the proximity of the drain or in the vicinity of the embeddedinsulating film.

Furthermore, a method of manufacturing a semiconductor integratedcircuit, in which a CMOS transistor is formed on a first conductivitytype semiconductor film provided on a first conductivity type supportingsubstrate through an embedded insulating film, includes the steps of:conducting thermal oxidation so as to reach the embedded insulating filmto form a LOCOS for element separation between transistors; forming agate oxide film of a first conductivity type transistor; forming a firstsecond conductivity type impurity region reaching the embeddedinsulating film on the semiconductor film in a region where the firstsecond conductivity type transistor is to be formed; forming a firstconductivity type impurity region having a higher density than that ofthe first second conductivity type impurity region in a portion of thesemiconductor film serving as the proximal region to a drain in thefirst conductivity type impurity region; forming a polysilicon filmserving as a gate electrode of the first conductivity type transistorand forming a second conductivity type impurity region; forming a secondconductivity type impurity region in each of the source region and thedrain region; forming an interlayer insulating film and forming contactholes in the source region, the drain region, and the gate electrode;and forming a wiring on the interlayer insulating film. In a transistorformed by the above-mentioned method, the depletion layer is extendedtoward the body side in the portion having a low density of the firstconductivity type impurity region in the proximity of the drain whilebeing extended toward the drain side in the portion having a highdensity so as to allow the uniformization of the extension of thedepletion layer in the proximity of the drain. As a result, thegeneration of impact ions can be reduced.

1. A method of manufacturing a semiconductor integrated circuit, incircuit in which a CMOS transistor is formed on a first conductivitytype semiconductor film provided on a first conductivity type supportingsubstrate through an embedded insulating film, comprising the steps of:conducting thermal oxidation to form a LOCOS for element separationbetween transistors in the semiconductor film; forming a gate oxide filmof a first second conductivity type transistor; forming a firstconductivity type impurity region between the gate oxide film and theembedded insulating film in a region where the first second conductivitytype transistor is to be formed; forming a polysilicon film on the gateoxide film and etching the polysilicon film so as to form a gateelectrode of the first second conductivity type transistor; forming asecond conductivity type impurity region in an ultra-shallow portion ofeach of a source region and a drain region; forming a secondconductivity type impurity region having a low density in a middleportion of each of the source region and the drain region; forming asecond conductivity type impurity region having the same density as thesecond conductivity type impurity region in the ultra-shallow portion ina lower portion of each of the source region and the drain region;forming an insulating film on the source region, the drain region, andthe gate electrode; dry etching the insulating film formed on the sourceregion, the drain region, and the gate electrode to form a sidewallaround the gate electrode; and performing ion implantation using thesidewall as a mask so as to form a second conductivity type impurityregion in each of the source region and the drain region.